Surface morphology and thickness of a multilayer film composed of strong and weak polyelectrolytes: Effect of the number of adsorbed layers, concentration and type of salts

Self-assembled multilayered films were prepared by alternate deposition of a strong cationic polyelectrolyte, poly(trimethylammonium ethyl methacrylate chloride) and a pH-dependant anionic polyelectrolyte, poly(acrylic acid). The layer-by-layer adsorption was followed in-situ by optical fixed-angle reflectometry and after drying by ellipsometry. A recently developed “substrate thickness method” was applied to calculate the adsorbed amount of polymer from the reflectometric signal. Surface film morphology was imaged before and after drying with atomic force microscopy (AFM). Influence of the number of adsorbed layers, concentration and type of salts on the multilayer growth was examined. The number of adsorbed layers produced a specific effect on the reflectometric signal which is linked to the refractive index of the film. Adjustment of the adsorbed amount of polyelectrolytes was done by changing sodium chloride salt concentration within a range of 10−3 to 10−1 M. AFM observations showed a significant evolution in surface morphology and a maximum of surface roughness for films built-up at 10−2 M. Experiments were then carried out at 10−3 M in either barium chloride or zinc chloride salts. In the presence of Ba2+ and Zn2+ ions, adsorption of 5 bilayers is completely modified and the surface morphology was smoother than the multilayers obtained using sodium chloride salt.